One of the most compelling potential uses of magnetic resonance spectroscopy (MRS) is as a noninvasive biochemical assay for key endogenous compounds. Small-scale studies suggest that quantitative MRS (qMRS), in which the results are reported as concentrations, can monitor biomarkers associated with diagnosis and treatment monitoring for diseases such as cancer, lupus, chronic pain syndrome, chronic fatigue syndrome, liver cirrhosis and cardiomyopathy. Robust qMRS requires compensation for the numerous scale factors introduced during signal excitation, acquisition and processing. Existing methods that compensate for these scale factors require acute diligence and are impractical for all but the most meticulous research sites. Existing methods also rely on questionable assumptions and/or lead to painful reductions in signal to noise ratios. As a result, nearly all MRS results are presented in terms of arbitrary units or as ratios, which can be ambiguous or misleading. Among the most onerous scale factors to compensate for are variations in RF coil loading conditions and coil reception sensitivity. We have developed a method that eases the burden of qMRS. Using single-channel surface coils we have previously demonstrated that this method is suitable for in vivo use in humans and automatically compensates for coil loading conditions. In this project we will translate the method to state-of-the-art, multi-channel phased array coils and enhance the algorithm to include compensation for RF coil reception sensitivity. Our detailed derivation of the enhanced algorithm shows that it can be implemented using off-the-shelf phased arrays without modification. All that is needed is a low-cost, discrete electro-optical device that operates completely independently from the magnet system software and hardware. The two functions of this device are to inject a stable, precalibrated artificial signal into the magnet's built-in body coil and to measure relative electrical current flow in the body coil. These features will allow qMRS to be integrated with chemical shift imaging sequences that take advantage of accelerated image acquisition protocols. Successful completion of this project will allow us to provide efficient, accurate, quantitative, noninvasive concentration maps of key endogenous compounds in human brains using unmodified phased-arrays and standard pulse sequences with no signal-to-noise ratio penalties, and virtually no increase in acquisition times compared to methods that report the results as arbitrary units.